Catheterization of human veins with needles and catheters is a common medical procedure. Clinicians frequently need to access patients' veins in order to draw blood for laboratory testing or for placement of intravenous (IV) catheters, for the administration of medicines, fluids or blood.
Catheterization is typically accomplished by placing a rubber tourniquet around an extremity, for example a forearm, proximal to the planned point of catheterization. The tourniquet causes compression of the superficial veins without compressing the associated arteries. Therefore, the blood is pumped through the arteries past the tourniquet into the distal extremity. Since the veins are compressed, the blood is prevented from returning to the heart. The veins typically dilate due to the increased intravascular pressure and are thus more visible and easier to access with the needle or catheter. Once the dilated vein is identified, the skin is cleaned and usually numbed with a local anesthetic. The needle or catheter is then inserted into the dilated vein.
Catheterization can be difficult to accomplish in infants and children, obese patients, patients with darker skin, IV drug abusers and patients receiving chemotherapy for cancer. Additionally, any patient can be difficult to cannulate if he or she is cold, frightened, apprehensive or dehydrated. This commonly occurs in patients that are injured or are about to undergo surgery. In these situations, veins are actively constricted by the sympathetic nervous system and, therefore, will not dilate in response to an increase in intravenous pressure. Even the application of a tourniquet may not cause the veins to visibly dilate.
It has been known that application of heat to the skin of a forearm helps to reduce vasoconstriction and dilate veins. Traditionally, heat has been applied by soaking towels in warm water and then wrapping the towel around a forearm. However, the use of wet towels has several significant drawbacks. The wet towels quickly cool. The wet skin experiences an evaporative heat loss that may actually cool the skin. The water is messy and may cause the skin to macerate. Therefore, the use of we towels has many significant deficiencies in effectiveness and convenience.
Electric heating pads have also been used for heating the skin of the forearm to aid IV catheterization. Electric heating pads do not have the cooling and messy problems associated with wet towels. However, electric heating pads may not be hot enough to achieve rapid vasodilation. If they are hot enough, the high temperature may inadvertently be applied for too long, risking thermal injury. Electric heating pads are difficult to wrap snuggly around the forearm and, therefore, typically do not maintain good contact with the skin for optimal conductive heat transfer.
Forced air patient blankets such as the Bair Hugger® blanket (distributed by Arizant Inc., Eden Prairie, Minn.), have also been used to warm the arms of patients for starting IVs. Such blankets are wrapped around a patient and then inflated with warm, forced air. However, the warm air cools inside of the blanket and does not remain warm enough to cause rapid vasodilation. Clinicians have attempted to avoid this cooling of forced air by blowing warm air directly onto a patient without using a blanket (a process known as “hosing”). However, hosing is not recommended because the direct contact of warm air with skin increases the risk of thermal burns. Moreover, the forced air is supplied by noisy blowers that are relatively energy-inefficient and complicated.
A loose fitting mitt made of carbon fiber conductive fabric has also been used. Lenhardt et al. published a study (in British Medical Journal 325:409, August 2002) that evaluated the effectiveness of such a loose fitting mitt. The mitt was heated to 52° C. and applied to patients for 15 minutes prior to starting an IV. The success rate for catheterization was 94% compared to 72% for an unheated control group. Warming the hand and forearm with a loose fitting mitt appeared to be useful for improving the success rate of catheterization. However, the required 15 minutes of warming time may be too long to be practical in clinical settings. In addition, an increase in temperature could subject the patients to a risk for thermal burn injuries. Additionally, the loose fitting mitt does not optimize conductive heat transfer to the skin because it does not conform to the skin to maximize skin-heater contact.
The prior art heaters have several drawbacks. Some of the heaters have rigid structures or loose fitting structures, which are undesirable because they do not conform to a patient's extremity. For example, a patient with a smaller arm may not have very much skin in contact with a loose fitting or rigid heater. This prevents optimal heat conductive heat transfer between the heater and the skin. Some of the heaters are also unnecessarily complicated and include a variety of support structures, chambers, forced air devices, suction devices and the like. Complicated heaters are difficult to apply to a patient during a clinical setting and delay the overall dilation time. Complicated heaters may also intimidate a patient.
Prior art heaters either do not heat at temperatures or lengths of time appropriate for rapid venous dilation, which is beneficial in a clinical setting. For instance, some prior art heaters heat at too low of temperatures or for insufficient lengths of time. The heaters also do not include a timing device to limit the duration of exposure to heat, thereby increasing the risk of thermal injury to a patient's skin. In addition, some of the heaters are provided in direct contact with a patient's extremity and must be cleaned between patients to avoid cross-contamination. A patient's bodily fluids may contaminate the heater and must be cleaned before the heater is used on the next patient. The cleaning of medical equipment is both expensive and inefficient.
There is a need for an improved heater that is simple to apply, is easy to clean, achieves rapid dilation, optimizes heat transfer, and/or reduces the risk of thermal injury. Certain embodiments of the invention described below solve one or more of the limitations of the prior art described above.